In recent years, in order to reduce the weight of the body of a car to increase fuel efficiency of automobiles, for example, high-strength steel sheets have been used for suspension members. In addition, since the regulation on collision safety has become stronger, there are needs to use high-strength steel sheets also for members having complex shapes, which have been made of low-strength steel sheets only. However, in general, as the strength of the steel sheet becomes higher, the ductility thereof is decreased and the formability degrades. Accordingly, in order to use the high-strength steel sheet for a member having a complex shape, it is necessary to manufacture a steel sheet that has both high formability and high strength. In particular, since a hot-rolled high-strength steel sheet is subjected to stretch-flanging to be used for suspension members of automobiles, stamped hole expandability is important as the index of strength and stretch-flanging. The stamped hole expandability of a steel sheet can be evaluated by an evaluation method regulated in ISO 16630.
Patent Document 1 discloses a method of performing cooling, immediately after final rolling at a temperature of lower than “Ar3 point+100° C.”, at an average cooling speed of greater than or equal to 400° C./second until the temperature becomes “Ar3 point−100° C.” so as to refine ferrite grains and obtain a strong <111> texture. This method improves the ductility and stretch-flanging property, resulting in improvement in the anisotropy of mechanical characteristics.
Patent Document 2 suppresses rolling textures and refines colonies having the rolling textures by increasing the final temperature of the hot-rolling after the addition of B. Further, by performing quenching at a temperature of higher than or equal to the lowest cooling temperature that is decided by the B amount in a run out table, recrystallization of austenite is promoted, the {110} surface strength of the rolling textures is decreased, and expansion of inclusions and ferrite crystal grains is suppressed. Thus, a method having high hole expandability and suppressing variations is proposed.
In a similar manner, as a technique of increasing the hole expandability while the strength of the steel sheet is increased, for example, Patent Document 3 discloses a technique of improving the balance between the strength and the hole expandability by optimizing the fraction of a steel structure of ferrite, bainite, or the like, and precipitates in the ferrite structure. However, the technique of Patent Document 3 does not have a sufficient hole expansion value, and does not have a sufficient balance between the strength and the hole expandability. In contrast, Patent Document 4 reveals that the hole expandability of the hot-rolled steel sheet can be improved by reducing the X-ray random intensity ratio in the {211} plane that is parallel to the rolled surface by using V as an essential element. Further, Patent Document 4 reveals that as the final rolling ending temperature is higher in the hot-rolling process, the X-ray random intensity ratio in the {211} plane is reduced more.